A receptor for the opsonic fragment of C3, C3b, in the human renal glomerulus has been further defined. It has been localized to the surface of the subepithelial cell of the glomerulus. Its importance in human autoimmune disease is suggested by its saturation in vivo with complement fragments in patients with autoimmune renal disease. An interstitial renal receptor for the Fc fragment of human and rabbit IgG antibody has now also been demonstrated. The function of this receptor is to be studied. A model of pneumococcal septicemia in the guinea pig has been established which allows one to examine the pathophysiological aspects of complement function. The overriding importance of the alternative complement pathway has been established and studies are underway to determine its mechanism of action in pathophysiologic terms. Studies of the interaction of complement components on cell surfaces have led to the development of a new assay for C3 inactivator. BIBLIOGRAPHIC REFERENCES: Shin, M. L., Gelfand, M. C., Nagle, R. B., Carlo, J. R., Green, I., and Frank, M. M.: Localization of receptors for activator complement of visceral epithelial cells of the human renal glomerulus. J. Immunol. 118: 869-873, 1977. Braylan, R. C., Jaffe, E. S., Mann, R. B., Frank, M. M., and Berard, C. W.: Surface receptors of human neoplastic lymphoreticular cells. In Thierfelder, S., Rodt, H., and Thiel, E. (Eds.): Blood (Supplement): Immunologic Diagnosis of Leukemias and Lymphomas. New York, Springer Verlag, 1977, pp. 47-54.